Iron is an essential nutrient for all living things; a large number of cellular enzymes and other proteins require iron in order to function properly. Although iron is amongst the most plentiful of metals, it is difficult for biological systems to acquire; in aerobic environments of substantially neutral pH, iron exists as its oxidized Fe.sup.3+ form which readily hydrates to highly insoluble Fe(OH).sub.3 polymeric forms. To ensure accessability of iron in their environment, aerobic and facultative micro-organisms synthesize and release into their environment highly selective iron chelating agents called siderophores, the function of which is to provide the microbes with this vital nutrient. The siderophores released by the microbes solubilize iron, putting it into a form readily usable by them. Thus, a free, microbial siderophore is a growth promoting substance for those organisms which can utilise the particular siderophore in question.